An image forming apparatus, which forms an image on a print medium, may be classified broadly into an electrophotographic type, in which a series of charging—light exposure—developing with developer—transferring—fusing—cleaning processes are performed to form an image on a print medium; an inkjet type where small ink droplets are selectively ejected from nozzles onto a print medium to form an image; and a thermal transfer type where a thermal print head is used.
In the case of the electrophotographic type image forming apparatus, toners of yellow (Y), magenta (M), cyan (C), and black (K) are applied to the print medium in a manner overlapping with one another to form a desired color image. Such a color electrophotographic type image forming apparatus may include a plurality of developing cartridges corresponding to the respective colors.
Each developing cartridge may include a photosensitive body on which an electrostatic latent image is formed, a developing roller which develops the electrostatic latent image with toner of the color corresponding to the particular developing cartridge, and a supplying roller which supplies the toner to the developing roller.
On the surfaces of the photosensitive bodies, visible images are developed with the toners of the respective colors by the developing rollers. In order to obtain a color image of good quality, the visible images have to be precisely applied at the correct positions so that when the visible images are overlapped with one another, the resulting color image shows a good alignment between the individual visible images or a good color registration as it is sometimes referred to.
The quality of the color image is affected by an error in color registration. That is, the less the error in the color registration, the higher the quality of the color image can be. One measure of the error in the color registration may be considered as the maximum spatial deviation of the individual dots of all four colors (Y, M, C, K) typically used to form a full color image.
Many image forming apparatus include components or subunits that can be detached from and reinstalled in the main body of the image forming apparatus so that these detachable units can be repaired, replenished and/or replaced. One such detachable unit that may also be a possible source of, or that may play some role in, color registration error may be the detachable developing cartridge that houses therein the photosensitive body or bodies. When a detachable developing cartridge is mounted in the main body of the image forming apparatus, a passive coupler at the end of a rotating shaft of the photosensitive body and a driving coupler at the end of a driving shaft provided in the main body become engaged with each other so that the rotational driving force can be transmitted to the photosensitive body. The passive coupler and the driving coupler engage in a face-to-face contact with each other to transmit the driving force.
Typically, the passive coupler has a plurality of first contact planes while the second coupler has a plurality of second contact planes. Corresponding ones of these contact planes come into the face-to-face contact with each other with the surface contact force acting along the direction parallel to the rotating shaft of the photosensitive body.
As the coupling of the passive coupler and the driving coupler is based primarily on surface contact where the surface contact pressure acts along the rotational axial direction, the driving shaft and the rotating shaft of the photosensitive body may be driven in the state in which they are misaligned with each other due to the fabrication and assembly tolerance, the vibration during operation, or the like. As a consequence, a change in the contact position or even no contact may occur between the first contact planes and the second contact planes respectively of the passive coupler and the driving coupler. Such misalignment in or lack of contact between the contact planes may cause the photosensitive body to rotate at a rotational speed that is not constant, but which varies in a periodic pattern, for example, as illustrated in FIGS. 1 and 2.
FIG. 1 plots the results of an experiment, in which, using a conventional image forming apparatus, and for each of the colors Y, M, C and K, a line of one hundred test points or dots is printed along a lengthwise direction of a print medium, and, for each of such test points, the positional error, i.e., the deviation distance, is measured. In FIG. 1, the X-axis represents the lengthwise direction of the print medium (i.e., the vertical scanning direction or the direction of travel of the print medium) while the Y-axis represents the positional error of the dots. Further, in FIG. 1, X=0 and X=250 indicate the leading and trailing edges of the print medium, respectively.
Plotted in FIG. 2 are the maximum spatial error (i.e., color registration error) among the four color lines at each test point from the results of FIG. 1. As can be observed from the plots, the color registration error varies, fluctuating periodically for several periods even during printing of a single page. The test results also show that the maximum color registration error observed was about 210 μm, and that color registration errors of no less than 126 μm (i.e., the approximate combined width of three dots for a resolution of 600 dpi) occurred at 41.4% of the test points. Accordingly, an improvement of the color registration is desired.